1. Field of the Invention
This invention relates to an apparatus for reading/writing an optical record carrier such as an optical disk. The apparatus comprises a radiation source such as a semiconductor laser diode, an objective lens focusing a light beam from the radiation source on the record carrier, and a focusing detection system which converts the light beam reflected from the record carrier into an electrical signal and produces therefrom a focusing error signal.
2. Description of the Prior Art
Such apparatus is known from the U.S. Pat. No. 4,051,527, issued Sept. 27, 1977. The apparatus derives a focus error signal from a far field image of an information structure, which is typically composed of a plurality of pits aligned along a spiral track formed on the record carrier. The focusing detection system comprises a photo sensor divided into two separate elements and a phase detection circuit. The photo sensor is disposed in the far field of the information structure at one side of a plane defined by the optical axis of the objective lens and a line normal to the information track. When the information structure of the optical disk is around the focal point of the objective lens, the reflected light beam is diffracted to a zero-order beam and higher-order beams. In the far field area, those diffraction beams, overlapping one another, produce bright and dark interference stripes the period of which is a function of the focus error. Furthermore, when the record carrier is rotating, the lines also move in the parallel direction to the projected track. The direction of the movement depends upon whether the focus error is positive or negative. For example, when the focus error is positive, that is, distance between the information structure and a principal plane of the objective lens is longer than a focal length of the objective lens , the stripes move forward, i.e., in the direction of the disk rotation, and when negative, backward.
Such behavior of the stripes in the far field can be detected as a phase difference between outputs of the two sensor elements. They are located in the half area of the far field, and their divided line is vertical to the line which is parallel to the projected track. Therefore, when one of the elements detects one of the interference stripes as a variation of light power, the other detects the same one after a time lag which is approximately in proportion to the focusing error. Since each element converts a variation of the strength of light to an electric signal, the time lag is converted to a phase difference between the both signals. When the focusing error is very small, the detected phase difference is almost zero because the period of the stripes is larger enough than the width of each detector. But the larger the focusing error becomes, the bigger the phase difference becomes. When the period of the stripes is almost equal to the width of the elements, the phase difference becomes almost 180 degrees, which is the maximum focusing error which the system can detect. The phase detection circuit converts the phase difference to an electric signal, which becomes a focusing error signal after passing through a low-pass filter.
In the above-described priory art, however, the focusing error signal is susceptible to a defect on the surface of the record carrier. The optical disk comprises a recording material layer formed on a transparent substrate having a thickness of almost 1 mm. The light beam from the objective lens radiates the recording material layer through the substrate. The substrate protects the information on the recording material layer from destruction. Even though the surface of the substrate is slightly scratched, the information structure on the recording material layer is not affected because the information structure is in the near field region while the defect in the far field region. However, in the above-described system, the defect in the far field region affects the focusing error signal which is derived from the far field image of the information structure. For example, if there is a black spot on the illuminated region on the surface of the substrate, its image is projected on the sensor. Furthermore, since the optical disk is rotating, the image of the defect moves from the one element to the other element of the sensor. At this time, the sensor detects the defect as electric signals which have a phase difference. After all the phase difference causes a focusing disturbance.